Traditional way of estimating planetary ages may be flawed

Counting craters may not be the best way to work out the age of a planet, scientists say today in Nature. Rather like guessing a person's age by looking at how wrinkly they are, astronomers have traditionally worked out how old planets and moons are by the number of marks on the surface - the more craters, the more meteorites it has encountered and the older it is.

But a recent study of the craters on Jupiter's icy moon, Europa, suggests that at least 95% of the small ones are formed by secondary impacts, where fragments of rock have bounced out from the main impact. This is likely to be true for planets such as Mars, meaning that scientists may have been overestimating planetary age.

Edward Beirhaus, from Lockheed Martin in Colorado, and his colleagues studied the first high-resolution images of Europa taken from the Galileo spacecraft. "We noticed that many of Europa's small craters (less than 1km diameter) are spatially clustered," said Professor Beirhaus.

On planets such as Mars the crater patterns have been interfered with by preferential erosion or preservation, but on Europa the craters are relatively pristine. Scientists do not expect to see clustering patterns from meteorite impacts, since meteorites come from all directions. The only plausible explanation was that they were formed by lumps of rock thrown out from a huge meteorite strike.

"These secondary craters throw a monkey wrench into the planetary age estimation problem," said Prof Beirhaus.